1. Field of the Invention
The present invention relates to a process and an apparatus for uniformly heat-treating a substrate which has a film-forming composition thereon.
2. Discussion of the Related Art
There are known substrates having films or layers formed thereon of a given material such as a metallic or inorganic material. In the present specification, the term "film" and the term "layer" are used interchangeably, unless otherwise specified. Such substrates include glass substrates made of a glass material, typically, a soda-lime glass, and ceramic substrates made of a ceramic material, typically, alumina. A film or layer, which has a certain function, may be bonded to the substrate by fusion or melting of a glass bonding component or by softening, melting or sintering of the material per se. These substrates may be used for anode plates for vacuum fluorescent displays (VFD), plasma switching boards for plasma address liquid crystal displays (PALC), field-emission displays (FED) and other display devices, thick-film wiring boards, and various electronic devices such as thermal printer heads and image sensors. Generally, the substrates for these electronic devices are subjected to heat treatments at temperatures of about 500-650.degree. C. for the purpose of annealing the substrates per se or forming functional films with a glass material used as a bonding agent. Where the substrates are ceramic, the substrates are heat-treated at about 500-900.degree. C. for forming functional films with a glass material used as a bonding agent or for forming functional films of a metallic material by utilizing the fusion of the metallic material at the interface with the substrates.
Recently, there have been increasing requirements for increasing the number of conductive, resistive, dielectric and other layers or films formed in desired patterns, and for increasing the density of such layers or films. Further, there has been an increasing demand for display devices having a large-sized display screen, and an accordingly increasing requirement for increasing the size of the substrates for such large-sized display devices. To meet these requirements, it is required to form minutely patterned layers or films over a comparatively large area, particularly, on the substrates for the display devices. The substrates for electronic devices described above have patterned functional films having minute cells or cavities. To assure high dimensional and positional accuracy of these minute cells, the functional films should be patterned with an improved degree of uniformity. However, the above heat treatment or firing of the substrates has an influence on the quality of the substrates, which influence increases as the size of the substrates increases. Therefore, the heat treatment causes a variation in the quality of the products using the substrates, and provides some restrictions in the design of the products, or reduces the yield of the products. The quality variation may be a variation in the resistance value of a resistor film, a variation in the withstand voltage of a dielectric film, a variation in the thickness due to uneven ratio of removal of binders by firing of the dielectric film, a variation in the continuity or resistance of a conductive film, a variation in the ease of wire-bonding or sputtering on the conductive film.
Where the substrate suffers from a dimensional change due to expansion or shrinkage of its material upon heat treatment, it is difficult to accurately position the patterned functional films relative to each other, since each function film is fired after it is formed in a predetermined pattern. The uniformity and positioning accuracy of the patterned films tend to be deteriorated with an increase in the density (minuteness) and size of the substrate, whereby the yield ratio of the product is significantly lowered as the density or size of the substrate is increased. In the case of a substrate for a plasma display device having a screen size as large as 40 inches, for example, the causes for lowering the yield ratio may include: insufficient dimensional accuracy of multiple layers which form multiple cells; variation in the height and width dimensions of partition walls; variation in the resistance of resistor cells; variation in the withstand voltage of a dielectric layer; an overall dimensional variation; and inaccurate positioning of front and rear plates which form a discharge cell.